I have had some major problems with my 78 (350 automatic).
1. At start up a wall off smoke comes out of exhaust. I've been told its my valve seals.
2. Smoke comes in through a/c vents.
3. engine is misfiring causing flames to come out exhaust.

Is it time for a complete rebuild job or should i start looking for a new engine?
Or is there any other suggestions?

 

flames out of exhaust could be a rich mixture. The smoking (in the car) might be an exhaust leak. I have a feeling the car hasnt been started in a while... if so, do a tune up - but it might be too late for that. Any more info?

 

1) Valve seals
2) Probably a leaking heater core
3) Flames out the exhaust? Is it a full length exhaust system? That's a long way to be shooting flames. Your engine may be running way rich (as cpl suggested) and pouring raw fuel out the exhaust.
I would start with giving it a good tune up. After the initial start up, does the engine still smoke? Has the car been sitting for a prolonged period of time? That could contribute to the oil puffing issue. If that's the extent of it, and the engine isn't drinking oil like water, then that could be something you could live with for a while. And if the engine is in otherwise sound condition, you can change the seals with the engine in the car. The seals sometimes dry up and crack from old age before the engine's time is up, so a smoke screen at start up doesn't always mean serious engine problems.